Performance of communications systems is driven by many factors that relate to either signal fidelity or signal-to-noise ratio (SNR). Fidelity relates to the faithful reproduction of the shape of the signal. SNR relates to the noise and interference levels in the system that transmits the signal. Designers and operators of communications systems that include an element that repeats the signal, such as a satellite transponder, strive to minimize the degradations to fidelity and SNR that occur in the transmission system. One parameter of a communications system, among others, that characterizes degradation is linearity. Linearity is typically most affected by the amplifiers in a communications system, particularly the final amplifier in the systems, also known as the high-power amplifier (HPA). HPAs in satellite systems are typically either solid state power amplifiers (SSPAs) or travelling-wave tube amplifiers (TWTAs).
In linear communications systems, the output signal (y) is linearly proportional to the input signal (x), related by only a multiplier (e.g., an amplification factor, a) and a constant (e.g., a bias, b). A plot of the output signal versus the input signal would produce a straight line (e.g., y=ax+b). Actual and practical communication systems amplifiers are typically linear over a limited range of input signal levels. For example, in the amplitude transfer function graph that has input signal level on the horizontal axis and output signal level on the vertical axis, as the input signal increases past this linear region, the output signal starts to compress (e.g., have reduced amplification) and will typically saturate (e.g., reach a level in which the output no longer increases as the input level increases). Similar linearity issues affect the phase response of an amplifier as well.
While practical amplifiers cannot avoid this non-linear behavior, users and designers of communications systems strive to minimize the extent of the non-linear region of the amplitude and phase transfer functions, attempting to match the performance of a practical ideal amplifier, which is perfectly linear until saturation is reached. In a typical application, a target linearity for a particular type of communications signal is determined, and then the amplifier is backed-off (e.g., input signal is moved lower than saturation) to an operating point that enables amplifier linearity performance to match requirements. A more linear system allows the system to be backed-off less, therefore, enabling higher output power (which benefits SNR), saving critical power resources, or both.
One way to allow an amplifier to more closely match the performance of an ideal practical amplifier is to use a linearizer. A linearizer is typically utilized in the amplification stages prior to the final HPA, with the objective of attempting to pre-distort the signal in such a way that the signal passing through the linearizer and then the HPA more closely matches the performance of an ideal practical amplifier. The linearizer attempts to provide the inverse of the shape of the HPA amplitude and phase, thereby, yielding a more linear amplifier function. Current generation linearizers, generally devised in the 1990s, provide acceptable performance. However, new satellite applications are moving to signal types that require higher (e.g., ultra-high) linearity and higher power to preserve the fidelity of the signal without excessive back-off. These systems may typically employ higher-order modulation types (e.g., 16APSK, 32APSK . . . 256APSK).